As well known in the art, a LAN is a group of computers and associated devices that share a common communications line or wireless link and typically share the resources of a single processor or server within a small geographic area, for example, within an office building. Usually, the server has applications and data storage that are shared in common by multiple computer users. The major local area network technologies include Ethernet, Token Ring and FDDI (Fiber Distributed Data Interface). Another LAN technology, ARCNET, once the most commonly installed LAN technology, is still used in the industrial automation industry.
LAN's have particular applicability to telecommunications applications. A suite of application programs, or services, may be maintained on the LAN server and distributed or broadcasted to remote telephone subscribers. Users who request an application may download it once and then execute it from their local storage device. In telecommunications, a client may decide to subscribe to a call forwarding service which is sent to the user through the LAN connection and stored locally in the user's next generation telephone.
A VLAN is a group of end-stations typically on multiple physical LAN segments that can communicate as if they are on a common LAN segment. This is done by grouping paths sharing the common LAN segments according to each VLAN. Typically, VLANs are implemented using tags in message headers in order to delineate the VLANs.
Layer 2 Ethernet bridges and switches “learn” where their clients are located in a network. Each time a bridge receives a frame it looks for the source address of the frame in its forwarding table, which is a table maintained by the switch that shows the switch where to forward a frame sent to a particular destination. A switch's forwarding table can be treated as a list of the addresses of its current clients plus information about how to forward frames destined for those clients.
If the source address is not listed in the forwarding table the switch adds an entry to the table for it. The entry identifies the switch port on which the frame from the source address was received. A VLAN-aware switch also records the VLAN on which the frame was received. A switch that supports VLAN tags. A VLAN-aware switch is a switch that makes frame forwarding decisions based on VLAN tags in frame headers and adds and removes tags as required.
If an entry for the address already exists in the forwarding table, but is associated with a different port or port VLAN than the incoming message, the switch will update its forwarding table to reflect the port, or port VLAN, on which it received the frame. This learning process is a standard feature of Ethernet switches and bridges.
In any Ethernet network there must be exactly one active path between any pair of communicating stations. A station is a device that originates and receives messages on a network. If multiple paths exist between two stations then a loop will exist in the network that could lead to outages caused by broadcast storms. This is a problem which will be better appreciated with reference to FIG. 1.
In FIG. 1, there is shown a LAN 100 with end stations 101, 102, 103 and 104, which may be end terminals, computers, telephone end-device or any terminal for that matter. Switches 105, 106, 107 and 108 connect the various end stations. In FIG. 1, there are redundant paths 109 and 110 between End Station 1, 2 or 3 and end station 4 respectively. The problem with redundant paths is that the same message is sent redundantly over different paths to the end station. In large networks, this can quite easily become problematic and can overwhelm a network in a relatively short time.
Switches and bridges typically deal with redundant links by shutting down all but one of the links in the redundant set. Protocols such as the known Spanning Tree Protocol (STP) are employed to detect redundant paths through the network and to select redundant links to shutdown. Spanning Tree Protocols make all the switches responsible for identifying redundant paths in a network.